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Journal articleChakravorty D, Chaudhuri B, Hui S, 2018,
Voltage dependent loads can collectively provide a certain amount of power reserve (by virtue of the ability to change their power consumption within the stipulated voltage tolerance) which could be exploited for grid frequency regulation through voltage control at the substation/feeder or at the point ofload. The amount of such power reserve would vary with time of the day depending on the incidence of different types of voltage dependent loads and also the voltage profile across the feeders. It is important for the grid operators to know the aggregate power reserve from the voltage dependent loads during different times of the day in order to schedule other forms of reserves accordingly. This paper presents a methodology to estimate such power reserve from the measured power and voltage at the bulk supply points without knowing the actual distribution network topology and/or load profile of individual customers. The proposed method is applied to estimate the time variation of the aggregate reserve offered by the voltage dependent loads within the domestic sector in Great Britain (GB). Studies on astandard IEEE distribution network are presented to validate the estimated reserve margins under typical voltage profiles across the distribution feeders.
Journal articleYan S, Wang MH, Yang TB, et al., 2018,
Achieving multiple functions of 3-phase electric springs in unbalanced 3-phase power systems using the instantaneous power theory, IEEE Transactions on Power Electronics, Vol: 33, Pages: 5784-5795, ISSN: 0885-8993
IEEE Three-phase electric spring (3-ph ES) has recently been proposed as a fast demand response technology for applications in unbalanced power systems fed with a mixture of conventional and renewable power generation. Using the Instantaneous Power Theory as the theoretical framework, this paper presents the criteria and conditions for minimizing the average and oscillating power of the 3-ph ES for the first time. A detailed analysis of the use of 3-ph ES is included for providing multiple control objectives of voltage regulation and power balancing of the 3-ph power system, and minimization of the average and oscillating ac power of the ES. A corresponding control scheme implementable in a single controller is included and explained. The control scheme has been practically verified with experiments.
Journal articleChen B, Pin G, Ng WM, et al., 2018,
This paper presents an adaptive observer-based robust estimation methodology of the amplitudes, frequencies and phases of biased multi-sinusoidal signals in presence of bounded perturbations on the measurement. The parameters of the sinusoidal components are estimated on-line and the update laws are individually controlled by an excitation-based switching logic enabling the update of a parameter only when the measured signal is sufficiently informative. This way doing, the algorithm is able to tackle the problem of over-parametrization (i.e., when the internal model accounts for a number of sinusoids that is larger than the true spectral content) or temporarily fading sinusoidal components. The stability analysis proves the existence of a tuning parameter set for which the estimator's dynamics are input-to-state stable with respect to bounded measurement disturbances. The performance of the proposed estimation approach is evaluated and compared with other existing tools by extensive simulation trials and real-time experiments.
Journal articleMajumdar A, Pal BC, 2018,
This paper demonstrates a concept to detect bad data in state estimation when the leverage measurements are tampered with gross error. The concept is based on separating leverage measurements from non-leverage measurements by a technique called diagnostic robust generalized potential (DRGP), which also takes care of the masking or swamping effect, if any. The methodology then detects the erroneous measurements from the generalized studentized residuals (GSR). The effectiveness of the method is validated with a small illustrative example, standard IEEE 14-bus and 123-bus unbalanced network models and compared with the existing methods. The method is demonstrated to be potentially very useful to detect attacks in smart power grid targeting leverage points in the system.
Journal articleGu Y, Bottrell N, Green TC, 2018,
A reduced-order model that preserves physical meaning is important for generating insight in large-scale power system studies. The conventional model-order reduction for a multiple-timescale system is based on discarding states with fast (short timescale) dynamics. It has been successfully applied to synchronous machines, but is inaccurate when applied to power converters because the timescales of fast and slow states are not sufficiently separated. In the method proposed here, several fast states are at first discarded but a representation of their interaction with the slow states is added back. Recognizing that the fast states of many converters are linear allows well-developed linear system theories to be used to implement this concept. All the information of the original system relevant to system-wide dynamics, including nonlinearity, is preserved, which facilitates judgments on system stability and insight into control design. The method is tested on a converter-supplied mini power system and the comparison of analytical and experiment results confirms high preciseness in a broad range of conditions.
Journal articleDe Paola A, Angeli D, Strbac G, 2018,
This paper proposes a novel decentralized technique for efficient integration of flexible demand in the electricity market. The analysis focuses on price-responsive appliances that schedule their power consumption on the basis of a demand/price signal received by a central entity. Previous work has shown that, when the devices population is sufficiently large to be described as a continuum, it is possible to provide necessary and sufficient conditions for the existence of a Nash equilibrium (no device has unilateral interest in changing its scheduling when considering the resulting profile of aggregate demand). These results are now extended in order to achieve an equilibrium also when the mentioned conditions are violated. To this purpose, a time-varying proportional constraint (equal for all devices) is introduced on the power rate of the price-responsive appliances so as to limit the variation of flexible demand that they can introduce at critical time instants. The proposed design technique not only guarantees existence of a Nash equilibrium but it also minimizes the global operation time of the appliances population. Simulation results are provided and it is shown that, under the considered assumptions, each individual appliance completes its task in minimum time.
Journal articleJunyent Ferre A, Li Y, Rodríguez-Bernuz JM, 2018,
A new three-phase active rectifier topology is proposed for bipolar dc distribution, which can achieve the independent dc-pole control, with only one two-level voltage source converter and an ac-side grounding inductor. The averaged large-signal model and linearized small-signal model of the rectifier are derived in the stationary reference frame. Moreover, a control system is proposed with proper controller parameters. Besides, the rectifier is tested on an experiment platform. Comprehensive experiment results are given and analyzed to validate the function of the proposed rectifier under different operation conditions, including the rectifier start-up performance, rectifier dynamics with unbalanced dc loads for two poles, and rectifier dynamics with asymmetrical dc voltages for two poles. Finally, the proposed rectifier is compared with other two existing ac-dc conversion approaches, in terms of required number and rating of components as well as power losses with different load imbalance levels, which further highlight some potential benefits of the proposed topology.
Conference paperMartinez Sanz IM, Chaudhuri B, Majumder R, 2018,
Use of HVDC links for direct connection of a largenuclear power station to the existing power grid is unprecedented.Control of such HVDC connection for stable and secure operationof the nuclear power station is challenging and untested. Forthe Moorside nuclear power station planned close to the LakeDistrict in Great Britain, there is tremendous pressure to useHVDC cables for the southern connection route (which wouldpass through the picturesque landscape) to avoid the visualimpact of overhead lines. This paper shows that it is feasibleto connect a large nuclear power station, such as Moorside,through a VSC-HVDC cable route alongside a northern ACroute where there are no objections to overhead transmission.Use of a proposed control of the VSC-HVDC is shown to achieveautomatic and rapid redirection of power on to the VSC-HVDClink following sudden outage of the AC route. Moreover, if thegenerators at Moorside were to shut down accidentally, the samecontrol strategy for VSC-HVDC can back feed the power stationauxiliaries which is essential for nuclear safety. Thus, a mix ofAC and HVDC connection routes achieves similar (if not better)transient responses as compared to using both AC routes whichin this case faces serious opposition.
Journal articleJudge PD, Merlin MMC, Green TC, et al., 2018,
Conference paperRodrigues T, Moreira R, Strbac G, 2018,
Recent developments on climate change regulations are leading to significant changes in the electricity industry with volatility levels and uncertainty increasing significantly. As aresult, systemoperators are extending the procurement of flexibility to end-users and encourage more pro-active behaviors, including provision of system balancing services. In this context, this paper will investigate the potential flexibility and benefits that a coordinated operation of multiple households’ rooftop solar and energy storage can offer to the various sectors of the electricity market. The model will analyze the coordinated operation of multiple households and minimize the supply costs for the whole community while offering flexibility services to the system operator and provide peak demand reduction at the distribution network level. Fundamentally, the proposed model will consider the possibility of households to share energy resources in a peer-to-peer arrangement and thus minimize energy supply costs. In addition, the model also considers the provision of frequency response and reserve services and their associated revenues.Our results have shown significant cost savings can be achieved through a coordinated energy trading and consequently reduce energy transactions with suppliers. Provision offlexibility throughbalancing services becomes the major benefit to support the business case for coordinated operation of storagein low carbon communities.
Conference paperPadoan A, Astolfi, 2018,
Model reduction by moment matching at isolated singularitiesfor linear systems: a geometric approach, 56th IEEE Conference on Decision and Control, Publisher: IEEE
The model reduction problem for continuous-time, linear, time-invariant systems is studied at isolated singularities of the transfer function. The moments at a pole of the transfer function are shown to be uniquely specified by the solutions of certain Sylvester equations exploiting two distinct approaches based on complex analysis and on geometric control theory. This allows to determine reduced order models which preserve given poles and match the corresponding moments. An in-depth analysis of the assumptions underlying this approach is provided in a companion paper. The applicability of the approaches developed is demonstrated with simple academic examples.
Conference paperDi Franco P, Scarciotti G, Astolfi A, 2018,
The problem of the stability analysis for constrained mechanical systems is addressed using tools from classical geometric control theory, such as the notion of zero dynamics. For the special case of linear constrained mechanical systems we show that stability is equivalent to a detectability property. The proposed techniques are illustrated by means of simple examples.
Journal articleMartinez Sanz I, Judge P, Spallarossa C, et al., 2017,
In future power systems, reduced overall inertia caused by an increased dominance of asynchronous generation and interconnections would make frequency control particularly challenging. As the number and power rating of voltage source converter (VSC) HVDC systems increases, network service provision would be expected from such systems and to do so would require overload capacity to be included in the converter specifications. This paper studies the provision of frequency services from modular multilevel converter (MMC)-based VSC HVDC interconnections using temperature-constrained overload capability. Overload of the MMC-based HVDC system is achieved through controlled circulating currents, at the expense of higher losses, and subject to a control scheme that dynamically limits the overload available in order to keep the semiconductor junction temperatures within operational limits. Two frequency control schemes that use the obtained overload capacity to provide frequency response during emergency conditions are investigated. The controllers' performance is demonstrated in the context of the future Great Britain transmission grid through a reduced equivalent test system. Simulation results show that even modest temperature margins which allow overload of MMC-based HVDC systems for a few seconds are effective as a primary frequency reserve and also reduce the loss of infeed requirements of such interconnections.
Journal articlePadoan A, Scarciotti G, Astolfi A, 2017,
A geometric characterization of the persistence of excitation condition for the solutions of autonomous systems, IEEE Transactions on Automatic Control, Vol: 62, Pages: 5666-5677, ISSN: 0018-9286
The persistence of excitation of signals generated by time-invariant, autonomous, linear, and nonlinear systems is studied using a geometric approach. A rank condition is shown to be equivalent, under certain assumptions, to the persistence of excitation of the solutions of the class of systems considered, both in the discrete-time and in the continuous-time settings. The rank condition is geometric in nature and can be checked a priori, i.e. without knowing explicitly the solutions of the system, for almost periodic systems. The significance of the ideas and tools presented is illustrated by means of simple examples. Applications to model reduction from input-output data and stability analysis of skew-symmetric systems are also discussed.
Journal articleKkelis G, Yates DC, Mitcheson PD, 2017,
This paper analyses and compares candidate zero dv/dt half-wave Class-E rectifier topologies for integration into multi-MHz inductive power transfer (IPT) systems. Furthermore, a hybrid Class-E topology comprising advantageous properties from all existing Class-E half-wave zero dv/dt rectifiers is analysed for the first time. From the analysis, it is shown that the hybrid Class-E rectifier provides an extra degree of design freedom which enables optimal IPT operation over a wider range of operating conditions. Furthermore, it is shown that by designing both the hybrid and the current driven rectifiers to operate below resonance provides a low deviation input reactance and inherent output voltage regulation with duty cycle allowing efficient IPT operation over wider dc load range than would otherwise be achieved. A set of case studies demonstrated the following performances: 1) For a constant dc load resistance, a receiving end efficiency of 95% was achieved when utilising the hybrid rectifier, with a tolerance in required input resistance of 2.4% over the tested output power range (50W to 200W). 2) For a variable dc load in the range of 100% to 10%, the hybrid and current driven rectifiers presented an input reactance deviation less than 2% of the impedance of the magnetising inductance of the inductive link respectively and receiving end efficiencies greater than 90%. 3) For a constant current in the receiving coil, both the hybrid and the current driven rectifier achieve inherent output voltage regulation in the order of 3% and 8% of the nominal value respectively, for a variable dc load range from 100% to 10%.
Journal articleDe Paola A, Angeli D, Strbac G, 2017,
Journal articleTzelepis D, Rousis AO, Dysko A, et al., 2017,
Conference paperDi Franco P, Scarciotti G, Astolfi A, 2017,
The problem of the stability analysis for nonlinear differential-algebraic systems is addressed using tools from classical control theory Exploiting Lyapunov Direct Method we provide linear matrix inequalities to establish stability properties of this class of systems. In addition, interpreting the differential-algebraic system as the feedback interconnection of a dynamical system and an algebraic system, a sufficient stability condition has been derived using the small-gain theorem. The proposed techniques are illustrated by means of simple examples.
Conference paperPudjianto D, Strbac G, Boyer D, 2017,
Virtual power plant: managing synergies and conflicts between transmission system operator and distribution system operator control objectives, CIRED 24th International Conference on Electricity Distribution, Publisher: IET, Pages: 2049-2052, ISSN: 2515-0855
In this study, the implementation of virtual power plant (VPP) as a means to coordinate the use of distributed resources for different control objectives by transmission system operator and distribution system operator is described. In order to illustrate the concept, a range of illustrative studies demonstrating the application of VPP concept on a real 11 kV system in Brixton will be presented, using data from the Low Carbon London project. The studies demonstrate the changes in the operating characteristics of the VPP area over a range of system operating conditions.
Conference paperHuyghues-Beaufond N, Jakeman A, Tindemans S, et al., 2017,
Challenges in model and data merging for the implementation of a distribution network contingency analysis tool, 24th International Conference & Exhibition on Electricity Distribution (CIRED), Publisher: IET, Pages: 1621-1624, ISSN: 2515-0855
The electricity network in the South East of England has become more challenging to manage both for the transmission and distribution network operators due to increased distributed generation connection and increased power flows on transmission interconnectors to and from continental Europe. UK Power Networks (UKPN), the distribution network operator (DNO), has trialled for the first time online contingency analysis on a distribution network in Great Britain. The Kent Active System Management project aims to demonstrate the benefits of using a contingency analysis system for both operational and planning time frames. This study describes challenges and the recommended approach to overcome data exchange and data-quality challenges when developing a real-time power flow model from existing datasets. It provides a real-world example of dealing with data exchange and also highlights the need for transmission system operator/DNO coordination.
Journal articleThammawichai M, Kerrigan EC, 2017,
We propose three novel mathematical optimization formulations that solve the same two-type heterogeneous multiprocessor scheduling problem for a real-time taskset with hard constraints. Our formulations are based on a global scheduling scheme and a fluid model. The first formulation is a mixed-integer nonlinear program, since the scheduling problem is intuitively considered as an assignment problem. However, by changing the scheduling problem to first determine a task workload partition and then to find the execution order of all tasks, the computation time can be significantly reduced. Specifically, the workload partitioning problem can be formulated as a continuous nonlinear program for a system with continuous operating frequency, and as a continuous linear program for a practical system with a discrete speed level set. The latter problem can therefore be solved by an interior point method to any accuracy in polynomial time. The task ordering problem can be solved by an algorithm with a complexity that is linear in the total number of tasks. The work is evaluated against existing global energy/feasibility optimal workload allocation formulations. The results illustrate that our algorithms are both feasibility optimal and energy optimal for both implicit and constrained deadline tasksets. Specifically, our algorithm can achieve up to 40% energy saving for some simulated tasksets with constrained deadlines. The benefit of our formulation compared with existing work is that our algorithms can solve a more general class of scheduling problems due to incorporating a scheduling dynamic model in the formulations and allowing for a time-varying speed profile.
Journal articleXiang X, Zhang X, Chaffey G, et al., 2017,
An Isolated Resonant Mode Modular Converter with Flexible Modulation and Variety of Configurations for MVDC Application, IEEE Transactions on Power Delivery, Vol: 33, Pages: 508-519, ISSN: 0885-8977
The dc tap or dc transformer will play an important role in interfacing different voltages of dc links in dc grids. This paper presents an isolated resonant mode modular converter (RMMC) with flexible modulation and assorted configurations to satisfy a wide variety of interface requirements for medium voltage dc (MVDC) networks. The transformer-less RMMC, as introduced in the literature, implemented a restricted modulation scheme leading to a very limited range of step-ratio and the diode rectifier resulted in unidirectional power flow. Both of these limitations are removed in this proposal and galvanic isolation has also been added. Moreover, this new RMMC approach can serve as a building block for variety of configurations. Two such derived topologies are given, which inherently balance the voltage and current between different constituent circuits and realize the high power rating conversion for very low or very high step-ratio application. The theoretical analysis is validated by a set of full-scale simulations and a down-scaled experimental prototype. The results illustrate that this isolated RMMC and its derivatives have promising features for dc taps or dc transformers in MVDC applications.
Journal articleCantoni M, Farokhi F, Kerrigan EC, et al., 2020,
Constrained finite-horizon linear-quadratic optimal control problems are studied within the context of discrete-time dynamics that arise from the series interconnec- tion of subsystems. A structured algorithm is devised for computing the Newton-like steps of primal-dual interior-point methods for solving a particular re-formulation of the problem as a quadratic program. This algorithm has the following properties: (i) the computation cost scales linearly in the number of subsystems along the cascade; and (ii) the computations can be distributed across a linear proces- sor network, with localized problem data dependencies between the processor nodes and low communication overhead. The computation cost of the approach, which is based on a fixed permutation of the primal and dual variables, scales cubically in the time horizon of the original optimal control problem. Limitations in these terms are explored as part of a numerical example. This example involves application of the main results to model data for the cascade dynamics of an automated irrigation channel in particular.
Journal articleJudge P, Chaffey G, Merlin MMC, et al., 2017,
The Hybrid MMC, comprising a mixture of fullbridgeand half-bridge sub-modules, provides tolerance to DCfaults without compromising the efficiency of the converter to alarge extent. The inclusion of full-bridges creates a new freedomover the choice of ratio of AC to DC voltage at which theconverter is operated, with resulting impact on the converter’sinternal voltage, current and energy deviation waveforms, allof which impact the design of the converter. A design methodaccounting for this, and allowing the required level of deratingof nominal sub-module voltage and up-rating of stackvoltage capability to ensure correct operation at the extremes ofthe operating envelope is presented. A mechanism is identifiedfor balancing the peak voltage that the full-bridge and halfbridgesub-modules experience over a cycle. Comparisons aremade between converters designed to block DC side faultsand converters that also add STATCOM capability. Resultsindicate that operating at a modulation index of 1.2 gives agood compromise between reduced power losses and additionalrequired sub-modules and semiconductor devices in the converter.The design method is verified against simulation results and theoperation of the converter at the proposed modulation index isdemonstrated at laboratory-scale.
Conference paperXiang X, Zhang X, Chaffey G, et al., 2017,
The dc-dc conversion will play an important role in multi-terminal dc networks and dc grids. This paper presents two isolated resonant modular multilevel converters (IRMMCs) to fulfill the large step-ratio conversion for medium voltage dc (MVDC) networks. The conventional resonant modular multilevel converters (RMMCs) suffer the common problems of non-isolation and high current stress, which are solved in the proposed IRMMCs. They not only inherit the beneficial features of inherent sub-module (SM) voltage-balancing and soft-switching operation from RMMCs, but also develop multi-module configurations to neutralize the current ripples on both sides of the dc-links. The theoretical analysis is verified by a set of full-scaled simulations for different application examples in MVDC collection and distribution. The results demonstrate the proposed IRMMCs and its derived configurations have good potential for operation as large step-ratio MVDC transformers.
Journal articleScarciotti G, Astolfi A, 2017,
Mathematical models are at the core of modern science and technology. An accurate description of behaviors, systems and processes often requires the use of complex models which are difficult to analyze and control. To facilitate analysis of and design for complex systems, model reduction theory and tools allow determining “simpler” models which preserve some of the features of the underlying complex description. A large variety of techniques, which can be distinguished depending on the features which are preserved in the reduction process, has been proposed to achieve this goal. One such a method is the moment matching approach.This monograph focuses on the problem of model reduction by moment matching for nonlinear systems. The central idea of the method is the preservation, for a prescribed class of inputs and under some technical assumptions, of the steady-state output response of the system to be reduced. We present the moment matching approach from this vantage point, covering the problems of model reduction for nonlinear systems, nonlinear time-delay systems, data-driven model reduction for nonlinear systems and model reduction for “discontinuous” input signals. Throughout the monograph linear systems, with their simple structure and strong properties, are used as a paradigm to facilitate understanding of the theory and provide foundation of the terminology and notation. The text is enriched by several numerical examples, physically motivated examples and with connections to well-established notions and tools, such as the phasor transform.
Journal articleMerlin MMC, Soto-Sanchez D, Judge PD, et al., 2017,
The extended overlap alternate arm converter: a voltage source converter with DC fault ride-through capability and a compact design, IEEE Transactions on Power Electronics, Vol: 33, Pages: 3898-3910, ISSN: 1941-0107
The Alternate Arm Converter (AAC) was one ofthe first modular converter topologies to feature DC-side faultride-through capability with only a small penalty in powerefficiency. However, the simple alternation of its arm conductionperiods (with an additional short overlap period) resulted in(i) substantial 6-pulse ripples in the DC current waveform,(ii) large DC-side filter requirements, and (iii) limited operatingarea close to an energy sweet-spot. This paper presents a newmode of operation called Extended Overlap (EO) based onthe extension of the overlap period to 60◦which facilitates afundamental redefinition of the working principles of the AAC.The EO-AAC has its DC current path decoupled from the ACcurrent paths, a fact allowing (i) smooth DC current waveforms,(ii) elimination of DC filters, and (iii) restriction lifting on thefeasible operating point. Analysis of this new mode and EO-AAC design criteria are presented and subsequently verifiedwith tests on an experimental prototype. Finally, a comparisonwith other modular converters demonstrates that the EO-AACis at least as power efficient as a hybrid MMC (i.e. a DC faultride-through capable MMC) while offering a smaller converterfootprint because of a reduced requirement for energy storagein the submodules and a reduced inductor volume.
Journal articleSingh AK, Pal BC, 2017,
This paper proposes a decentralized method fornonlinear control of oscillatory dynamics in power systems. Themethod is applicable for ensuring both transient stability as wellas small-signal stability. The method uses an optimal control lawwhich has been derived in the general framework of nonlinearcontrol using normal forms. The model used to derive the controllaw is the detailed subtransient model of synchronous machinesas recommended by IEEE. Minimal approximations have beenmade in either the derivation or the application of the controllaw. The developed method also requires the application ofdynamic state estimation technique. As the employed control andestimation schemes only need local measurements, the methodremains completely decentralized. The method has been demon-strated as an effective tool to prevent blackouts by simulating amajor disturbance in a benchmark power system model and itssubsequent control using the proposed method.
Journal articleMylvaganam T, Sassano M, Astolfi A, 2017,
A multi-agent system consisting of N agents is considered. The problem of steering each agent from its initial position to a desired goal while avoiding collisions with obstacles and other agents is studied. This problem, referred to as the multi-agent collision avoidance problem, is formulated as a differential game. Dynamic feedback strategies that approximate the feedback Nash equilibrium solutions of the differential game are constructed and it is shown that, provided certain assumptions are satisfied, these guarantee that the agents reach their targets while avoiding collisions.
Journal articleDjapic P, Strbac G, McKenna R, et al., 2018,
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